Image capturing devices include cameras, portable handheld electronic devices, and other electronic devices. These image capturing devices use an automatic exposure mechanism to automatically calculate and adjust exposure settings in order to match as closely as possible a subject's mid-tone to the mid-tone of the photograph. “Correct” exposure may be defined as an exposure that achieves the effect the photographer intended. The purpose of exposure adjustment (in combination with lighting adjustment) is to control the amount of light from the subject that is allowed to fall on the film or image sensor, so that it falls into an appropriate region of the characteristic curve of the film or image sensor and yields a “correct” or acceptable exposure. A photograph may be described as overexposed when it has a loss of highlight detail. This can result in the bright parts of an image being effectively all white. A photograph may be described as underexposed when it has a loss of shadow detail. This can result in the dark areas of the image being indistinguishable from black.
One approach for determining the exposure setting for an image on a bright sunny day is the Sunny f/16 rule. This rule is a method of estimating correct daylight exposures without a light meter. The basic rule for a sunny day sets aperture to f/16 and shutter speed to the ISO film speed. For example, for a sunny day with ISO film 100 in the camera, one sets the aperture to f/16 and the shutter speed to 1/100 or 1/125 (on most cameras 1/125 is the available setting nearest to 1/100 second). For a sunny day with ISO film 200 in the camera, one sets the aperture to f/16 and the shutter speed to 1/200 or 1/250. The aperture and shutter speed can be jointly adjusted to achieve the same exposure (e.g., for ISO film 200, the aperture setting of f/8 and shutter speed of 1/500 is equivalent to ISO 200, f/16 and 1/250). FIG. 1 illustrates applications of the Sunny f/16 rule for different film speeds and different shutter speeds. As with other light readings, shutter speed can be changed as long as the f-number is altered to compensate.